Soma has kind of dropped the ball on several of their frames. The San Marco and the Buena Vista mixte come designed for 26" or 650c wheels in the smaller sizes, but don't go down to really small sizes that very short adults could use.
Some manufacturers make high-end juvenile road bikes that might work for a really small adult as well. Don't know how easy it would be to find 24" road tires, but it's possible to get a Pinerello and an Argon 18. There may be others out there as well.
http://www.pinarello.com/eng/speedy_792.php
http://www.argon18bike.com/bicyles/j...xenon-650.html (This one has 650C wheels)
Kona makes a small version of the Jake 'cross bike.
http://www.konaworld.com/jake_24.cfm